This section provides background information related to the present disclosure which is not necessarily prior art.
Comprehensive two-dimensional gas chromatography (GC×GC) is an analytical technique used to separate and detect the components of complex mixtures of volatile organic compounds. Unlike standard gas chromatography, which uses a single column for vapor separation, GC×GC couples a first-dimension column to a relatively short second-dimension column whose retention properties are complementary to those of the first-dimension column. Through a junction-point modulator between the two columns, mixture components partially separated on the first dimension column are focused and reinjected as a series of narrow pulses onto the second-dimension column. This results in an increase in peak capacity (i.e., the number of compounds that can be separated in a given analysis) and in sensitivity as a 2-D chromatogram is produced. The term ‘comprehensive’ refers to the fact that none of the originally injected sample is lost during the modulated separation process.
A pneumatic or thermal modulator (TM) is used at the interface between the two columns, with the latter generally providing a greater degree of sensitivity enhancement. A thermal modulator relies on low temperature to trap and focus the analytes as they elute from the first-dimension column, and then reintroduces them to the second-dimension column by rapid heating. By repeating this operation in rapid succession, the vapor profile is parsed into several segments, each of which is eluted through the second-dimension column. Thermal modulators, which are used in conventional bench-scale GC×GC systems, can be grouped into a thick stationary-phase film modulator and a cryogenic modulator. In the former, analytes are focused in a small section of capillary by the polymeric phase at ambient temperature. The latter uses a cryogenically cooled fluid such as CO2, N2, or air.
The two types of thermal modulators commonly use conductive or convective heating techniques to rapidly raise the temperature of the modulator. However, those macro-scale thermal modulators relying on cryogenically cooled fluids are resource intensive and/or demand a large amount of refrigeration work (e.g., ˜10 kJ for a cooling cycle of ˜5 s). Furthermore, power dissipation for typical heating devices can be on the order of 1 kW. Numerous efforts have been made to develop gas chromatography prototypes containing microfabricated components (μGC). These miniaturized systems and their components can operate at relatively low power; however, the lengths of the columns employed are inherently limited, with typical columns ranging in length from 0.5 to 3 metres. Accordingly, this places an inherent limit on the peak capacity. A μGC system incorporating two-dimensional gas chromatography (i.e., μGC×μGC) is a promising approach to overcome this limitation. Although a preliminary report has recently been described on the use of pneumatic modulation in a μGC×μGC system, there has yet to be a report on a microfabricated thermal modulator (μTM) for μGC×μGC applications. To be effective, the microfabricated thermal modulator (μTM) must span a broad range of temperature (e.g. −50 to 250° C.) at a very high rate (>1000° C. s−1, ideally during both heating and cooling).